The present invention relates generally to digital receivers, and, more particularly, to a system, and an associated method therefor, for calculating channel gain and noise variance of a communication channel upon which an information signal is transmitted.
A communication system which transmits information between two locations includes, at a minimum, a transmitter and a receiver interconnected by a communication channel upon which an information signal (which contains information) may be transmitted.
A radio communication system comprises one type of communication system. The communication channel of a radio communication system is formed of a radio-frequency channel. The radio-frequency channel is defined by a range of frequencies of the electromagnetic frequency spectrum. To transmit an information signal upon the radio-frequency channel, the information signal must be converted into a form suitable for transmission thereof upon the radio-frequency channel.
Conversion of the information signal into the form suitable for the transmission thereof upon the radio-frequency communication channel is effectuated by a process referred to as modulation wherein the information signal is impressed upon a radio-frequency electromagnetic wave. The radio-frequency electromagnetic wave is of a characteristic frequency of a value within a range of values of frequencies which defines the radio-frequency channel. The radio-frequency electromagnetic wave of a characteristic frequency upon which the information signal is impressed is commonly referred to as a "carrier signal". The radio-frequency electromagnetic wave, once modulated by the information signal, is referred to as a modulated, information signal, or, more simply, a modulated signal. As the modulated signal contains the information to be transmitted between the transmitter and the receiver, the modulated signal is also commonly referred to as the communication signal.
The modulated signal is of a frequency bandwidth spanning a range of frequencies, sometimes referred to as the modulation spectrum of the modulated signal. The center frequency of the modulation spectrum is located at, or close to, the frequency of the carrier signal. Because the modulated signal may be transmitted through free space upon the radio-frequency channel, the transmitter and the receiver need not be positioned in close proximity to one another. As a result, radio communication systems are widely utilized to effectuate communication between two locations.
Many techniques for modulating an information signal to form the modulated signal thereby have been developed. Examples of such techniques include, for example, amplitude modulation (AM), frequency modulation (FM), phase modulation (PM), frequency-shift keying modulation (FSK), phase-shift keying modulation (PSK), and continuous phase modulation (CPM). One type of continuous phase modulation is Gaussian minimum shift keying modulation (GMSK). Another type of continuous phase modulation is quadrature amplitude modulation (QAM). A particular type of QAM modulation is filtered, differential quadrature phase shift keying (DQPSK) modulation.
The transmitter of the radio communication system contains circuitry for modulating an information signal according to a particular modulation technique, such as one of the techniques above-mentioned. The modulated signal formed thereby is transmitted upon the radio-frequency channel, and is received by the receiver of the communication system.
The receiver contains circuitry to detect, or to recreate otherwise, the information signal of the modulated signal transmitted thereto upon the communication channel. Such circuitry performs a process, referred to as demodulation, which is essentially the reverse of the modulation process.
Demodulation techniques have been developed analogous to corresponding ones of the modulation techniques to detect, or to recreate otherwise, the information content of a modulated signal. The circuitry of the receiver must be of a construction to demodulate a modulated signal by a demodulation technique corresponding to the modulation technique by which the modulated signal is formed by a transmitter which transmits the modulated signal thereto.
Typically, the circuitry of the receiver includes circuitry, sometimes consisting of several stages, to convert downward in frequency the modulated signal transmitted upon the communication channel.
Modulated signals generated by many different transmitters may be simultaneously transmitted as long as the simultaneously-transmitted, modulated signals do not overlap in frequency. By modulating information signals upon carrier signals of dissimilar frequencies, the modulated spectra of the resultant, modulated signals formed thereby are of bandwidths of frequencies to prevent such overlap.
Regulatory bodies have divided portions of the electromagnetic frequency spectrum into frequency bands, and have regulated transmission of modulated signals upon various ones of the frequency bands. The frequency bands have further been divided into channels, and such channels form the radio-frequency channels of a radio communication system. Regulation of the transmission of modulated signals within various ones of the frequency bands, and, more particularly, upon the channels into which the frequency bands have been divided, minimizes interference between simultaneously-transmitted, modulated signals.
For example, portions of a 100 megahertz frequency band extending between 800 megahertz and 900 megahertz are allocated in the United States for radio telephone communications. Such radio telephone communication includes radio telephone communication in a cellular, communication system. Conventionally, a radio telephone contains circuitry to permit simultaneous generation and reception of modulated signals, to permit thereby two-way communication between the radio telephone and a remotely-located transceiver.
A cellular, communication system is formed by positioning numerous base stations at spaced-apart locations throughout a geographical area. Each base station contains circuitry to receive modulated signals transmitted by one, or many, radio telephones.
The positioning of each of the base stations forming the cellular, communication system is carefully selected to ensure that at least one base station is positioned to receive a modulated signal transmitted by a radio telephone positioned at any location throughout the geographical area. That is to say, at least one base station must be within the transmission range of a radio telephone positioned at any such location throughout the geographical area. (Because the maximum signal strength, and, hence, maximum transmission range, of a signal transmitted by a base station is typically greater than the maximum transmission range, of a signal generated by a radio telephone, the maximum transmission range of a signal generated by a radio telephone is the primary factor which must be considered when positioning the base stations of the cellular, communication system.)
Because of the spaced-apart nature of the positioning of the base stations, portions of the geographical area throughout which the base stations are located are associated with individual ones of the base stations. Portions of the geographical area proximate to each of the spaced-apart base stations define "cells" wherein a plurality of cells, each associated with a base station, together form the geographical area encompassed by the cellular, communication system. A radio telephone positioned within the boundaries of any of the cells of the cellular, communication system may transmit, and receive, modulated signals to, and from, at least one base station.
Increased usage of cellular, communication systems has resulted, in many instances, in the full utilization of every available transmission channel of the frequency band allocated for radio telephone communication. As a result, various ideas have been proposed to utilize more fully the frequency band allocated for radio telephone communications. By more efficiently utilizing the frequency band allocated for radio telephone communication, the transmission capacity of an existing cellular, communication system may be increased.
The transmission capacity of the cellular, communication system may be increased by minimizing the modulation spectrum of the modulated signal transmitted by a transmitter. By reducing the bandwidth of the modulation spectrum, the radio-frequency channels upon which the modulated signals are transmitted may be reduced, thereby permitting a greater number of radio-frequency channels to be defined over a given frequency band.
Additionally, the transmission capacity of the cellular, communication system may be increased by minimizing the amount of time required to transmit a modulated signal. By minimizing the amount of time required to transmit the modulated signal, a greater number of modulated signals may be sequentially transmitted over a single radio-frequency channel.
By converting an information signal into discrete form prior to modulation thereof, and then modulating the discrete, information signal, the resultant, modulated signal is typically of a smaller modulation spectrum than a corresponding modulated signal comprised of an information signal that has not been converted into discrete form. Additionally, when the information signal is converted into discrete form prior to modulation thereof, the resultant, modulated signal may be transmitted in short bursts, and more than one modulated signal may be transmitted sequentially upon a single transmission channel.
Converting the information signal into discrete form is typically effectuated by an encoding technique, and apparatus which effectuates such conversion is referred to as an encoder. An encoded signal generated as a result of such an encoding technique may, for example, be in the form of a discrete binary data stream. The elements (i.e., bits) of the discrete binary data stream represents various characteristics of the information signal. The binary data stream comprising the coded signal may be appropriately filtered, and modulated by a modulation technique, as noted hereinabove, to form a modulated signal of a frequency appropriate for transmission upon a particular communication channel.
Transmission errors resulting in inaccurate detection or recreation of the information signal transmitted upon the transmission channel are primarily caused by three factors: 1.) spurious noise; 2.) intersymbol interference, and 3.) Rayleigh fading.
Spurious noise includes noise generated within electrical circuitry, such as thermal noise, as well as noise caused by transient signals or overlapping signals transmitted upon adjacent communication channels.
Intersymbol interference is caused by reflection of a single, transmitted signal off of man-made and/or natural objects. Although only a single modulated signal is generated and transmitted by a transmitter, the signal received by the receiver is actually the summation of a plurality of signals transmitted to the receiver over a plurality of signal paths. An actual (i.e., nonideal) radio-frequency channel upon which a signal is transmitted is therefore sometimes referred to as a multipath channel as a signal transmitted upon the channel is received by a receiver after transmission thereto over the plurality of different signal paths.
Transmission of the signal over any but a direct path results in propagation delay, and such propagation delay results in a receiver receiving the same signal, but delayed in time responsive to the signal path of the transmitted signal during transmission of the signal upon the radio-frequency channel. As the signal paths can be of various path lengths, a receiver actually receives the same signal a plurality of times corresponding to the plurality of paths of the multipath channel.
Significant propagation delay results in a signal delay resulting in interference between sequentially transmitted signal bits of the transmitted signal. Such interference is intersymbol interference.
Rayleigh fading is associated with intersymbol interference in that Rayleigh fading is caused by reception of a modulated signal transmitted over a plurality of channels. Propagation delays of time periods not great enough to cause intersymbol interference do, however, cause variance in magnitude and phase of the signal level received by the receiver. Such variance in magnitude and phase is Rayleigh fading.
Intersymbol interference and Rayleigh fading degrade receiver performance. Receiver performance is, at least in part dependent upon a channel gain characteristic of the channel upon which a signal is transmitted. Channel gain is a relative value which is representative of the magnitude of a signal received by a receiver (and is, hence, also representative of the attenuation of a signal transmitted upon the channel). Receiver performance is also dependent, in part, upon the noise variance of the signal received by a receiver. Noise variance is a statistical property of the magnitude of a noise component, which includes spurious noise, of a signal received by a receiver. Both channel gain and noise variance are time varying values, and values of channel gain and noise variance are additionally dependent upon levels of intersymbol interference and Rayleigh fading.
Several existing receiver constructions include circuitry to attempt to correct for intersymbol interference and Rayleigh fading to minimize receiver error resulting therefrom. For instance, one such existing receiver construction attempts to correct for such transmission error by altering the value of the received signal by a constant factor. Such an attempt, in essence, assumes that channel gain and noise variance are constant values. Therefore, such an attempt to correct for transmission error is inherently erroneous as the channel gain and noise variance of the communication channel are not constant values.
Another existing receiver construction attempts to correct for such transmission error by estimating the magnitude of the channel gain by first measuring the variance of the received signal and then calculating a square root of the measured variance of the received signal. Such an estimation is inherently erroneous when the noise variance is not relatively constant.
What is needed, therefore, is an improved means for determining the values of the actual channel gain of a signal transmitted upon a communication channel.